The present invention relates generally to the measurement of digital transmission links and, in particular, to bit-error structure measurements of data transmission channels using quasirandom sequences as a measuring signal, with inserted marking in the form of bit inversions at defined sample points.
Data measuring devices are known which employ quasirandom sequences (QRSs) as measuring signals. Having been transmitted over an object under test (data transmission link), the transmitted QRSs are compared in the data measuring receiver to an identically generated QRS reference signal.
The result of this comparison is a binary signal sequence which indicates for each bit whether it was correctly or incorrectly transmitted. This bit error signal represents the bit error structure which can be recorded as a numerical sequence of the consecutive correctly or incorrectly transmitted bits (see Herzog, W.: Ein Bitfehlerstruktur-Mexcex2gerxc3xa4tfxc3xcr den Betrieb. [A bit error structure measuring device for the operation.] telekom praxis, Vol. 74/1997, No. 11), which is hereby incorporated by reference herein.
Further known is the input at the transmission end of a marking at a fixed sample point of the QRS. The marking is composed of a test error series of systematic bit inversions which is inserted regularly, but not at each QRS cycle. The test error series includes a higher number of bits than the QRS-generating shift register has stages, so that it clearly differs from transmission errors caused by the transmission link.
Since, at the receiving end, the test error series appear as bit errors at a known sample point of the QRS, they can be used in the recorded bit error structure as a reference point for QRS sample calculations. This makes it possible to find systematic errors which point to device failures in the course of the transmission link under test. Improving quality by detecting and eliminating device failures is an objective of bit error structure measurement technology.
If the clock pulse interval of the test error series is monitored at the receiving end, said clock pulse interval having to correspond to a whole-number multiple of the QRS cycle length, then it is possible to calculate slips of the kind which occur when the character synchronism between test transmitter and test receiver is lost, e.g., as a result of cell losses in ATM (Asynchronous Transfer Mode) measurements.
In order to be able to use the advantages of this high-quality measuring method, the transmitting end must be provided with a suitable QRS test transmitter capable of transmitting test data with such a QRS marking. In fact, however, this is often not the case for measurements on transmission links if the transmitting end is merely provided with commercially available measuring devices which, although employing QRS signals, do not recognize a marking of the type described. However, the measuring devices are often used because of other measuring advantages.
Such measuring advantages may lie in the fact that more meaningful end-to-end measurements are possible instead of loop measurements, or that commercially available measuring devices supply the appropriate measuring interfaces at the transmitting end, and no new development is then necessary at the bit error structure measuring location.
Practical cases of different, usually commercially available, measuring devices at the transmitting end arise, firstly, in the event of large geographical separation between transmitter and receiver in worldwide data traffic (transmitter in California, receiver in Berlin) and, secondly, in the case of measurements between connection points with different interfaces (ATM transmission at the in-house rate of 25.6 Mbit/s for the transmitter and at the line-side STM1 rate, usual for ATM measurements, of 155.52 Mbit/s for the receiver).
Since a measuring device used at the transmitting end often does not allow optional input of external test data which could be provided with a QRS marking in a manner suitable for the process, it is necessary to make do with those QRS measurement data which are provided internally by the measuring device itself.
However, the bit error structure measuring location used at the receiving end will then not have the-test error series which is required later as a QRS mark in the evaluation of the bit error structure records.
An object of the present invention is to replace a QRS marking missing at the transmitting end by process engineering in such a way that an equivalent evaluation is possible for the bit error structure.
The present invention provides a method for bit error structure measurements of a data transmission channel of a data transmission link, the method including transmitting a quasirandom sequence from a transmitting end over the transmission link to a receiving end, the quasirandom sequence serving as a measuring signal, and marking the transmitted quasirandom sequence at the receiving end upstream of a bit-for-bit comparator so as to form bit inversions at defined sample points, the marking being synchronized with the transmitted quasirandom sequence at the receiving end.
The present invention also provides a system for bit error structure measurements of a data transmission channel of a data transmission link, the system including a transmitter for transmitting a quasirandom sequence from a transmitting end over the transmission link to a receiving end, the quasirandom sequence serving as a measuring signal, and a marker for marking the transmitted quasirandom sequence at the receiving end upstream of a bit-for-bit comparator so as to form bit inversions at defined sample points, the marking being synchronized with the transmitted quasirandom sequence at the receiving end.
According to the present invention a substitute marker is created at the receiving end of bit error structure measuring locations by additionally disposing a QRS symbol detector, a QRS starting pulse generator and a QRS marker, which is switchable from case to case, at the receiver input of the data measuring device before the bit-for-bit comparator.